Grain oriented sheet metal



United States Patent C) 3,096,222 GRAIN ORIENTED SHEET METAL Howard C. Fiedler, Nislrayuna, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Original application Aug. 5, 1958, Ser. No.

753,181. Divided and this application Apr. 27, 1961,

Ser. No. 105,900

3 Claims. ((31. 148-3155) This invention relates to magnetizable iron and related alloys such as those used in transformers, motors, etc., and more particularly to polycrystalline sheet-like bodies composed principally of an alloy of iron and silicon having additions of vanadium to promote formation of a preferred grain orientation.

This application is a division of applicants copending application, Serial No. 753,181, filed August 5, 1958, now abandoned, and assigned to the same assignee as the present application.

The sheet materials to which this invention is directed are usually referred to in the art as electrical silicon steels or, more properly, silicon-irons and are conventionally composed of iron alloyed with about 1.5 to 4 percent and preferably about 2.5 to 3.5 percent siliconand relatively minor amounts of various impurities such as sulfur, manganese, phosphorus and having low carbon content as finished material.

Such alloys crystallize in the body-centered cubic crystallographic system at room temperature. As is well known, this crystallographic arrangement refers to the symmetrical distribution or arrangement which the atoms forming the individual crystals or grains assume in such materials. The body-centered cube is composed of four atoms, each arranged at the corners of the unit cube with the remaining atoms positioned at the geometric center. Each unit cell in a given grain or crystal in these materials is substantially identical in shape and orientation with every other unit cell comprising the grain.

The unit cells or body-centered unit cubes comprising these materials each have a high degree of magnetic aniso tropy with respect to the crystallographic planes and directions of the unit cube and, therefore, each grain or crystal comprising a plurality of such unit cells exhibits a similar anisotropy. The silicon iron allays to which this invention is directed are known to have their easiest direction of magnetization parallel to the unit cube edges, their next easiest direction perpendicular to a plane passed through diagonally-opposite parallel unit cube edges, and their least easiest direction of magnetization perpendicular to a plane passed through a pair of diagonally-opposite atoms in a first unit cube face, the central atom and a single atom in the unit cube face which is parallel to the first face.

It has been found that these silicon-iron alloys may be fabricated by unidirectional rolling and heat treatment to form sheet or strip material composed of a plurality of crystals or grains, a majority of which have their atoms arranged so that their crystallographic planes have a similar or substantially identical orientation to the plane of the sheet or strip and to a single direction in said plane. This material is usually referred to as oriented or grain oriented silicon-iron sheet or strip and is characterized by having 50 percent or more of its constituent grains oriented so that four of the cube edges of unit cells of the grains are substantially parallel to the plane of the sheet or strip and to the direction in which it was rolled and a (110) crystallographic plane substantially parallel to the plane of the sheet.

It will thus be seen that these so-oriented grains have a' direction of easiest magnetization in the plane of the sheet in a rolling direction and the next easiest direction "ice of magnetization in the plane of the sheet in the transverse-to-rolling direction. This is conventionally referred to as cube-on-edge orientation or the (110) [001] texture. In these polycrystalline sheet and strip materials it is desirable to have as high a degree of grain orientation as is attainable in order that the magnetic properties in the plane of the sheet and in the rolling direction may approach the maximum attained in single crystals in the direction.

Strip and sheet grain oriented silicon-iron alloys have been previously used as transformer core materials, electric motor and generator laminations and in other electrical and electronic applications "where the high degree of electromagnetic properties in the rolling direction of the sheet or strip may be advantageously employed. For most applications, the highest degree of grain orientation or texture obtainable is desirable. Usually, materials having more than about 70 percent of their crystal structures oniented in the [001] texture are considered to have a strong texture.

Heretofore, the cube-on-edge texture has been produced in silicon-iron alloys by adding controlled amounts of manganese and sulfur, introduced into the mate-rial as impurities. The manganese and sulfur are probably present as a dispersion of manganese sulfide following a recrystallization phase which occurs during an annealing treatment. The significance of the manganese sulfide is demonstrated by the fact that strong textures cannot be developed in high purity silicon-iron alloys prepared from vacuum-melted, substantially pure iron, silicon and carbon.

Before these silicon-irons may be used in certain applications, such as the motors, generators, etc., mentioned tabove, i-t is necessary to remove substantially all the sulfur in order to attain optimum magnetic properties, since the presence of sulfur exerts an adverse effect upon the magnetic properties.

In actual steel mill practice, cube-on-edge materials are prepared by casting ingots from alloys containing from about 2.5 to 4.0 percent and preferably from 2.5 to 3.5 percent by Weight silicon, less than 0.035 percent carbon, about 0.02 to 0.03 percent sulfur, and less than 0.15 percent manganese. These ingots are conventionally hot' worked into a strip or sheet-like configuration, usually less than 0.150 inch in thickness referred to as hot rolled band.

The hot rolled band is then cold rolled with appropriate annealing treatment to the finished sheet or strip thickness, usually involving at least a 50 percent reduction in thickness and given a final or texture producing annealing treatment. As presently practiced, this final anneal is accomplished in two steps. First, a short normalizing anneal is carried out at about 800 C. for about 5 minutes in a wet hydrogen or wet cracked gas atmosphere. This anneal serves at least two purposes. It decarburizes the material or, stated otherwise, reduces the carbon content of the material to a value of less than 0.030 percent by weight, and additionally causes the worked metal structure to recrystallize into a time grain microstructure. This is usually referred to as a primary recrystallization. Because of the relatively low temperature and short time involved in this anneal, it is possible to employ a continuous annealing technique wherein the sheet or strip of metal is fed through a controlled atmosphere furnace at a rate such that any given portion of the strip is raised to the required temperature for the necessary period of time. Such continuous annealing techniques are Widely employed in the metallurgical arts' and are usually more economical than batch anneals.

The carburized strips or sheets are then cooled and coated with a refractory mate-rial and, depending upon their size and configuration, either coiled or stacked and placed in an enclosed box which is provided with an atmosphere of dry hydrogen or dry cracked gas or in a controlled atmosphere furnace and annealed therein. During this anneal, two actions occur. First, a secondary recrystallization takes place wherein the small grains having the desired (110) [001] orientation grow at the expense of grains having other orientations and, secondly, the sulfur content is lowered and preferably substantially removed. As conventionally commercially practiced, it has been found necessary to anneal such material over a considerable period of time in order to accomplish the two action-s previously stated and to produce acceptably strong textures. This has required the employment of a batch-type anneal, the total time required for such annealing usuallyrequiring from one to two days, since in order to accomplish the anneal in the most economical fashion, large amounts of metal are annealed in each batch.

After annealing, the sheet or strip material must then be flattened to remove warping which usually occurs during the final anneal. This is accomplished by heating the strip or sheet and applying tension thereto, according to existing stretch leveling practices.

The principal difi'lculty encountered in the use of sulfur is that of removing it from the alloy. Also, the desired degree of texture cannot be obtained if more than about 5.1 percent silicon is used. At present, the alloy must be heat treated at moderately high temperatures, e.g., 1175 C. for several hours. Obviously, extreme length of the heating period materially increases the cost of producing the final sheet, both through the cost of the operation itself and by resulting in a batch process.

It is a principal object of this invention to provide an iron-base silicon alloy which can be processed in shorter periods of time than can existing iron-silicon alloys to produce sheet material having a preferred cube-on-edge grain orientation.

Another object of this invention is to provide an ironbase silicon alloy containing sufficient amounts of a vanadium carbide second phase to promote development of a preferred cube-on-edge grain orientation and to provide for rapid removal of the second phase after development of the preferred orientation.

Other objects and advantages of the present invention will be in part obvious and in part explained by reference to the accompanying specification.

Briefly stated, the present invention utilizes relatively small additions of vanadium, present as vanadium carbide, to control efiectively the secondary recrystallization of the silicon-iron alloys without sulfur being present and to provide for easy removal of the additions, thus improving the magnetic characteristics of the alloy.

More specifically, it has been found that relatively minor additions of vanadium, for example, 0.50 to 2.0 percent, to the iron-silicon alloy will form a second phase precipitate of vanadium carbide and fix the grain boundaries, thereby preventing normal grain growth during annealing steps. Generally, additions of vanadium are preferred to be in the vicinity of 0.50 percent, although additions up to 2.0 percent are acceptable. The alloys of the present invention contain, in addition to the vanadium, from 1.5 to 6.0 percent silicon, up to about 0.050 percent carbon, and the balance iron. Of course, nitrogen, oxygen, sulfur and manganese are present but these elements are preferably reduced to trace percentages.

'When the alloy is subjected to the orienting annealing operation, grain growth proceeds rapidly in the preferred direction, at the expense of the normal grain growth which was inhibited by the dispersed vanadium carbide inclusions.

The procedure followed in the present invention to produce a silicon-iron body having the desired orientation is to cast molten metal containing as little sulfur and manganese as possible, into ingot or slab form. It will be appreciated that a trace of sulfur and manganese will probably be found in the alloy due to impurities in the raw materials or from the refractory furnace crucible.

Upon solidification of the metal, it is hot rolled to about 100 mil thickness, this particular thickness usually being referred to as the hot rolled band. The hot rolled band is annealed, permitted to cool and then cold rolled to within the range of thicknesses of from 0.029 inch to about 0.025 inch and then given an intermediate normalizing heat treatment. The metal is then cold rolled to 12 to 14 mil thickness, and the final annealing done to effect secondary recrystallization, that is, to bring on the (110) [001] orientation. The final anneal is advantageously carried out at somewhere between 950 C. and 1050 C.

It has been found that the atmosphere or environment in which the metal is subjected to the final texture-developing anneal is important to the attainment of optimum texture. Since the purpose of the carbide second phase is present to restrict normal grain growth, removal of the phase prior to development of the desired texture permits normal grain growth and thereby lowers the amount of desired texture obtained. Thus, the sheets or strips of material are preferably annealed in a non-decarburizing environment, such as a vacuum or a hydrogen-methane mixture which is essentially neutral or slightly carburizing rather than in a decarburizing atmosphere such as pure hydrogen. Methane-hydrogen atmospheres having from 200 to parts of hydrogen per part of methane have proven acceptable, with a ratio of about 100 parts hydrogen to 1 part methane being preferred.

Lastly, a purification step to remove vanadium carbide is carried out in a decarburizing hydrogen atmosphere at temperatures in excess of 1050 C., a preferred range being from 1050 C. to 1200 C. The particular temperature used is not critical, although, generally, slightly higher decarburizing times must be provided as lower temperatures are used. The length of the purifying time is not critical, since periods ranging from 15 minutes to 3 hours are sufiicient to effect removal of the second phase carbide. For example, 15 minutes at 1050 C. in hydrogen with a dew point of 30 F., reduces the carbon content from 0.04 percent to 0.003 percent. This purification causes dissociation of the vanadium carbide and provides for the carbon diffusion toward the surface of the metal for removal by the surrounding atmosphere. Since carbon diffuses through the iron about 200 times faster than sulfur at 1100 C., the iron can be purified more quickly when vanadium carbide is used as the second phase material. Thus, the oriented or textured iron can be more easily, quickly and cheaply produced.

A number of heats of different alloys were made, of which the following listed compositions are representative.

Table I Heat Percent Percent Percent Percent Percent Percent Si O V S 0 N Strip material Was prepared from the preceding alloys by heating the ingots to about 1000 C. and rolling without reheating to strip or hot rolled band mils thick. This rolled material was then annealed at 900 C. for /2 hour in dry (dew point about 60 F.) hydrogen to effect complete recrystallization. However, this anneal may be omitted if desired, and an atmosphere other than hydrogen may be used. The annealed bands were then cold rolled to 25 mil thickness and annealed at 860 C. for 2 minutes in dry hydrogen, then cold rolled to 13 mil thickness. It should be noted that this intermediate annealing temperature is not critical but should be maintained between about 850 C. and 950 C. for optimum results,

Specimens of this cold rolled strip or sheet material were then subjected to a texture-developing anneal comprising heating for between 2 and 4 hours at about 950 C. in vacuum. The annealing time and percent cube-onagent retarding normal grain growth. The percentage compositions of two such alloys and the texture developed are shown in the following Table IV:

edge texture in each of these samples is indicated in the 5 Table IV following, Table ll, the percent texture being calculated from torque values observed when the samples were sus- Heat t 51 3 M11 0 V 0 N Texture pended in the field of a magnetometer:

4 5.78 .002 trace .047 .72 .010 .002 84 Table II 5 5.69 .002 trace .050 .48 .008 .001 84 Heat figg Higher silicon alloys have lower magnetostriction and (hrs) lower watt losses and are therefore particularly valuable for use in applications, such as in transformers and the 1. 4 77 15 like, where electrical hum or vibration are troublesome. gg 3; Alloys containing from 5.1 to 6.0 percent silicon can be produced in the same manner as that outlined in con- Table III shows the percent texture developed in sam- Demon Wlth. the lower slhcon the excepfwn 1 16s treated in pure hydrogen and in various rnethane- 0 $3 3 ronmg temperatures must be 200 to 300 q z iig at 1000 for 20 to mmute Thus, the present invention makes it possible to propane S 0 1 T ble HI duce electrical grade silicon-iron alloys, having a direca tion of preferred grain orientation, more quickly and economically than has previously been possible. Also, Percent Texture higher silicon percentages can be alloyed with the iron Heat Atmosphere and still develop the proper degree of orientation of the grams. H2 HzzCHr/ H2ZCH4/10021 HflZOH4/5 I What I claim as new and desire to secure by Letters It can be seen that pure hydrogen alone does not develop the same degree of texture as the hydrogen-methane mixtures, probably due to partial decarburizing of the alloy. As a consequence, normal grain growth competes with secondary recrystallization and the full texture is never developed. By adding enough methane so that the atmosphere is neutral or even slightly carburizing, normal grain growth is completely prevented and secondary recrystallization goes to completion.

Three strips 0.0125 to 0.013 inch thick were made from heat 3 of Table I in the same manner as previously set forth, except that the purifying was carried out at 1100" C. for /2 hour. The electrical losses were found to be 0.63 watt per pound, which compares favorably with the previously discussed sample and with ordinary mill-made material.

Additional heats of silicon-iron alloys containing more than about 5 .1 percent silicon were produced and, through use of suitable additions of vanadium, high texture percentages were obtained. As already mentioned, a high degree of cubeon-edge texture has not previously been obtainable in silicon-iron alloys having above about 5.1 percent silicon, when manganese sulfide is used as the Patent of the United States is:

1. A polycrystalline sheet-like body having a majority of the constituent grains oriented in the [001] direction consisting essentially of from 1.5 to 6.0 percent silicon, an eifective amount up to about 0.050 percent carbon combined with from 0.50 to 2.0 percent vanadium in the form of a vanadium carbide second phase dispersion effectively promoting said (110) [001] grain orientation direction, and the balance substantially all iron.

2. A sheet-like body as defined in claim 1 wherein said silicon is present in amounts of from about 5.1 to 6.0 percent.

3. A polycrystalline sheet-like body having a majority of the constituent grains oriented in the (110) [001] direction consisting essentially of from 2.5 to 3.5 percent silicon, an effective amount up to about 0.05 percent carbon combined with from 0.50 to 2.0 percent vanadium in the form of a vanadium carbide second phase dispersion eifectively promoting said (110) [001] grain orientation direction, and the balance substantially all iron.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A POLYCRYSTALLINE SHEET-LIKE BODY HAVING A MAJORITY OF THE CONSTITUENT GRAINS ORIENTED IN THE (110) 001! DIRECTION CONSISTING ESSENTIALLY OF FROM 1.5 TO 6.0 PERCENT SILICON, AN EFFECTIVE AMOUNT UP TO ABOUT 0.050 PERCENT CARBON COMBINED WITH FROM 0.50 TO 2.0 PERCENT VANADIUM IN THE FORM OF A VANADIUM CARBIDE SECOND PHASE DISPERSION EFFECTIVELY PROMOTING SAID (110) 001! GRAIN ORIENTATION DIRECTION, AND THE BALANCE SUBSTANTIALLY ALL IRON. 